SAG meal penalties incur every half hour after six hours from previous meal. There is a 15 minute grace period, so if you're just ten minutes over, it's no big deal. Afterwards, though, it's $25 each performer for each of the first half hours, then it's $50 for each half hour. On our shoot we kept two SAG actors an hour and a half over lunch -- that's three meal penalties (2x25 and 1x50=$100 for each actor). Since this was an Ultra Low SAG project the actors were just making $100 a day. So by keeping them over lunch we essentially paid them for a whole extra day of shooting.
The penalties are the same for all SAG actors, from Tom Cruise to extras. Technically, your crew should be given a meal penalty too. I'll talk about crew unions later.
The penalty is paid to the actors, with the usual percentages taken out for their agents, SAG pension and health, etc.
Lesson: Lunch at six hours. End of story.
UPDATE: Hey, I just checked my Ultra Low Budget contract and realized that it's just $25 each half hour, it doesn't increase. Also you're to pay a percentage, so if you're only ten minutes after the grace period you just pay that percentage, whatever that would be. Now, each SAG contract probably handles this differently, so the above scenario is for Ultra Low Budget only.
The SAG website states the breakdown is thus for the basic agreement:
Check your own contract for specifics. It's there. Also, while they do expect you to pay P&H on meal penalties, they don't want it reported on the final cast list (for whatever reason -- probably because they don't assess dues from penalties).